1. Field of the Invention
The present invention relates to diamond film electronic devices using boron (B)-doped semiconducting diamond films, and its fabrication method.
2. Description of the Related Art
As a semiconducting material, diamond has a large band gap and an excellent stability against chemicals and radiation. Hence it is expected to be applicable for electronic devices used in harsh environments. In particular, thermistors using diamond films doped with boron (hereinafter, referred to as B-doped diamond films) are expected to be operated in a wide temperature range between room and high temperatures, and have a short response time and an excellent chemical resistance. Thus, attempts have been done to fabricate such thermistors (Fujimori, and others; NEW DIAMOND, vol 5, No. 2, p. 32, (1989), and Japanese Patent Laid-open No. Hei 3-83301).
FIG. 9 is a perspective view of a thermistor made by a prior art using B-doped diamond films. A B-doped diamond film 32 is formed on a substrate 31 made of Si.sub.3 N.sub.4. A pair of electrodes 33 are formed on the diamond film 32. The electrode 33 consists of a bilayer of Ti/Au films, or a multi-layer of Ti/Mo/Au films. Also, a protective layer 34, made of SiO.sub.2, is deposited between a pair of electrodes 33 in such a manner as to be slightly extended onto the electrodes 33. Furthermore, lead wires 35 made of Ni are attached to the areas of the electrodes 33, not covered with the protective layer 34, with Ag pastes 34.
The thermistor described above has a negative temperature coefficient, in which the resistance decreases with increasing temperature.
However, the above thermistors and other electronic device having similar structures have the following disadvantages: semiconducting diamond electronic devices are expected to be used at high temperatures and in other harsh environments. However, in the prior art of electronic devices using diamond films, the metal electrodes are exposed to air. Therefore, even though diamond films are not damaged in harsh environments, if electrode surfaces, electrode extraction portions, lead wires and the like are directly exposed to heat and external atmosphere, the electronic devices may be deteriorated.
To protect the electrode portions, the whole surface of the device may be covered with a protective layer, or the device may be enclosed in a protective container. However, if such methods are used for diamond film thermistors, for example, the temperature sensing part of the thermistor (i.e. B-doped diamond film) is separated from the environment and thus the thermal response time becomes extremely longer.
Furthermore, diamond film thermistors of the prior art have such a disadvantage that, in the mass-production, since only one set of the diamond film and electrodes is formed on each substrate, a large variation in the thermistor characteristics from one thermistor to another is unavoidable. Finally, since a set of the diamond film and electrodes is independently formed on each substrate, the productivity is low and hence the production cost is high.